Recently, from the viewpoint of increase in environmental consciousness, liquid fuels in which the contents of substances such as sulfur and aromatic hydrocarbons are small have been demanded. From such a viewpoint, as a technique that can produce a base stock for fuel oil that substantially contains neither sulfur nor aromatic hydrocarbon, and is rich in aliphatic hydrocarbons, and particularly a base stock for kerosene and gas oil, a technique in which a synthesis gas (mixed gas containing carbon monoxide gas and hydrogen gas as a main component) is produced from a hydrocarbon raw material such as natural gas by a reforming reaction, hydrocarbons are produced from the synthesis gas by a Fischer-Tropsch synthesis reaction (hereinafter, also referred to as an “FT synthesis reaction” in some cases.), and further the hydrocarbons are refined by hydroprocessing and fractionation to obtain a base stock for fuel oil has been attracting attention (for example, see Patent Literature 1.). The technique is referred to as a GTL (Gas To Liquids) process.
An synthetic oil obtained from the synthesis gas by the FT synthesis reaction (hereinafter, also referred to as an “FT synthetic oil” in some cases.) is a mixture containing aliphatic hydrocarbons having a wide carbon number distribution as a main component; the FT synthetic oil can be fractionated depending on a boiling point to obtain a naphtha fraction, a middle distillate fraction, and a wax fraction. Moreover, of the respective fractions, the middle distillate is the most useful fraction corresponding to the base stock for kerosene and gas oil, and it is desired that the middle distillate be obtained at a high yield.
A relative amount of the wax fraction contained with the middle distillate in the FT synthetic oil is hydrocracked; thereby, hydrocarbons whose boiling point is in the range of the boiling point equivalent to that of the middle distillate can be obtained; using the method, the useful middle distillate can be obtained from the FT synthetic oil at a high yield.
In hydrocracking of a hydrocarbon raw material oil containing the wax fraction, in order to increase the yield of the middle distillate, the cracking rate of the wax fraction needs to be increased; on the other hand, if the cracking rate is increased, there is a problem such that production of a light fraction due to excessive hydrocracking is increased while the yield of the middle distillate is reduced. Accordingly, it is demanded that a hydrocracking catalyst used for hydrocracking of a hydrocarbon raw material oil containing the wax fraction have a high hydrocracking activity, and simultaneously suppress production of a light fraction due to excessive hydrocracking to produce the middle distillate at a high selectivity. As such a hydrocracking catalyst, catalysts comprising: a catalyst support containing zeolite and an amorphous composite metal oxide having solid acidity; and an active metal supported by the catalyst support, selected from noble metals of Group 8 to Group 10 in the periodic table are known (for example, see Patent Literature 2 and Patent Literature 3.).
Usually, when the hydrocracking catalyst is packed in a reactor and operation of hydrocracking of the raw material oil containing the wax fraction is performed, the activity of the catalyst is reduced with a lapse of the operating time. Then, if the activity of the catalyst is reduced to a predetermined level, the operation of the hydrocracking step is stopped, and the catalyst is replaced. Moreover, the used hydrocracking catalyst taken out from the reactor (hereinafter, also referred to as a “used hydrocracking catalyst” or simply referred to as a “used catalyst” in some cases.) is regenerated and reused; thereby, cost needed for an expensive catalyst can be reduced, and disposal of the used catalyst as wastes can be avoided.
In the conventional regeneration of the hydrocracking catalyst, the used catalyst is calcined to remove substantially all the carbonaceous substances produced and deposited on the catalyst in the hydrocracking step, which are counted as a main cause of reduction in the activity.